Speedlite 550EX is the main component of a new flash system designed together with the EOS-3 SLR camera. It provides full compatibility with the new area AF technology employed by the EOS-3 and refined E-TTL autoflash for improved performance. Other main features include a maximum Guide Number of 180 (ISO 100, ft.), an AF-assist beam which links to the EOS-3's 45-point area AF, FP Flash (high speed sync), FE lock (a flash version of AE lock), and FEB (Flash Exposure Bracketing). The Speedlite 550EX also incorporates a built-in wireless transmitter, which can control other Speedlite 550EX units set up as slave units. Flash coverage is set automatically from 24mm to 105mm, and a wide-angle panel extends the coverage to 17mm. The new Speedlite runs on 4 AA-size batteries, and can also be used with optional external power supplies such as Compact Battery Pack E and Transistor Pack E. Recycling times are faster than those experienced with the Speedlite 540EZ. Speedlite 550EX is compatible with all EOS models.